The present invention relates generally to a variable speed drive for a chiller system. More specifically, the present invention relates to a variable speed drive with boost capability for a variable speed chiller system.
In the past, the motors for driving compressors in chiller systems were designed to operate from standard line (main) voltages and frequencies that were available from the power distribution system of the facility where the motor was being operated. The use of line voltages and frequencies typically limited the options for modulating the capacity of the compressors to less efficient mechanical devices such as inlet guide vanes and slide valves, as a result of the motor being limited to one operating speed that was based on the input frequency to the motor. In addition, if the operating speed of the motor was not equal to the desired operating speed of the compressor, a “step up,” or “step down,” gearbox was inserted between the motor and the compressor to obtain the desired operating speed of the compressor.
Next, variable speed drives (VSDs) were developed that could vary the frequency and voltage that was provided to the motors of a chiller system. This capability to vary the input frequency and voltage to a motor resulted in a motor that was capable of providing a variable output speed to the corresponding compressor of the chiller system. The variable speed operation of the motors (and compressors) enabled the chiller system to take advantage of efficiencies that occur during partial loading of the compressors, when operation at a speed lower than full load design speed is desirable. The use of the variable speed drive also permitted the use of other types of motors that required their own electronic drive in chiller systems in addition to the previous motors that were capable of operating directly from a three-phase power line, e.g., induction motors, or synchronous motors.
One limitation of prior induction motor style VSDs was that the magnitude of the fundamental output voltage from the VSD could be no larger than the magnitude of the input, or utility, line voltage to the VSD without creating an excessive amount of harmonic voltage. This limit on the fundamental output voltage occurred because the rectifier of the VSD only provided a DC voltage that is of a magnitude equal to approximately 1.3 times the root mean square (rms) value of the line-to-line AC voltage supplied to the VSD. This limitation on the output voltage of the variable speed drive limits the maximum speed of the conventional induction motor to a speed that corresponds to the speed of the motor operated at line voltage if constant volts/hertz ratio operation (also known as constant flux operation) was required. Constant volts/hertz ratio operation is needed by the conventional induction motor in order to deliver the motor's rated level of torque. Thus, to obtain greater compressor speeds, a “step up” gearing arrangement had to be incorporated between the motor and the compressor to increase the output rotational speed of the motor driving the compressor. Alternately one could operate a lower rated voltage motor at voltages and/or frequencies in excess of its rated voltage and frequency, provided the motor was capable of withstanding the rotational forces associated with such high-speed operation.
Prior induction motor style VSDs are also limited in their ability to provide ride-through capability that enables them to withstand temporary sags on the input line voltage. At full speed operation, the input line voltage sag was almost immediately reflected on the output voltage of the VSD, since the converter of the VSD contained no means to compensate for the input line voltage sag. The ability to withstand such temporary voltage sags of several seconds when operating at speeds close to maximum were thus limited, since the output voltage of the VSD is dependent upon the magnitude of the DC Link voltage, and thus the input voltage.
In addition, this limitation on the output voltage from the VSD limited the operating efficiency of high speed induction motors in the chiller system. The high speed motors, used to obtain faster compressor speeds without the “step up” gearing arrangement, were limited because it is more difficult to design an efficient and cost-effective motor when a reduced maximum operating voltage was available at the maximum operating speed.
Therefore, what is needed is a variable speed drive for a motor, with improved ride-through capability, and that can provide a cost-effective, efficient and easily implemented operation of the high speed induction motor in a chiller system.
What is also needed is a variable speed drive that is capable of driving a motor that is rated for operating voltages in excess of the nominal input line fixed AC voltage.